Organic reactions catalyzed by methylrhenium trioxide: Dehydration, amination, and disproportionation of alcohols

Article abstract of DOI:10.1021/jo951613a

Methylrhenium trioxide (MTO) is the first transition metal complex in trace quantity to catalyze the direct formation of ethers from alcohols. The reactions are independent of the solvents used: benzene, toluene, dichloromethane, chloroform, acetone, and in the alcohols themselves. Aromatic alcohols gave better yields than aliphatic. Reactions between two different alcohols could also be used to prepare unsymmetric ethers, the best yields being obtained when one of the alcohols is aromatic. MTO also catalyzes the dehydration of alcohols to form olefins at room temperature, aromatic alcohols proceeding in better yield. When primary (secondary) amines were used as the limiting reagent, direct amination of alcohols catalyzed by MTO gave good yields of the expected secondary (tertiary) amines at room temperature. Disproportionation of alcohols to alkanes and carbonyl compounds was also observed for aromatic alcohols in the presence of MTO. On the basis of the results of this investigation and a comparison with the interaction between MTO and water, a concerted process and a mechanism involving carbocation intermediates have been suggested.

Full text of DOI:10.1021/jo951613a

324
J . Org. Chem. 1996, 61, 324-328
Or ga n ic Rea ction s Ca ta lyzed by Meth ylr h en iu m Tr ioxid e:
Deh yd r a tion , Am in a tion , a n d Disp r op or tion a tion of Alcoh ols
Zuolin Zhu and J ames H. Espenson*
Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011
Received September 5, 1995X
Methylrhenium trioxide (MTO) is the first transition metal complex in trace quantity to catalyze
the direct formation of ethers from alcohols. The reactions are independent of the solvents used:
benzene, toluene, dichloromethane, chloroform, acetone, and in the alcohols themselves. Aromatic
alcohols gave better yields than aliphatic. Reactions between two different alcohols could also be
used to prepare unsymmetric ethers, the best yields being obtained when one of the alcohols is
aromatic. MTO also catalyzes the dehydration of alcohols to form olefins at room temperature,
aromatic alcohols proceeding in better yield. When primary (secondary) amines were used as the
limiting reagent, direct amination of alcohols catalyzed by MTO gave good yields of the expected
secondary (tertiary) amines at room temperature. Disproportionation of alcohols to alkanes and
carbonyl compounds was also observed for aromatic alcohols in the presence of MTO. On the basis
of the results of this investigation and a comparison with the interaction between MTO and water,
a concerted process and a mechanism involving carbocation intermediates have been suggested.
In tr od u ction
include heterogeneous and homogeneous reactions with
a stoichiometric amount of dehydrating agent, such as
anhydrous copper(II) sulfate,19 copper(II) sulfate on silica
gel,20 ferric chloride on silica gel,21 SOCl2/NEt3,22 TsOH/
PhH,23 BF3/OEt2,24 Ph3P/CCl4/NEt3,25 or Ph3PBiBr2/I2.26
The method reported here, which uses MTO as a catalyst
for the dehydration of alcohols at room temperature in
dry benzene, is more convenient.
Methylrhenium trioxide (CH3ReO3 or MTO) catalyzes
the epoxidation1 and metathesis2 of olefins, aldehyde
olefination,3 oxygen transfer,4 and the transfer of carbene
and nitrene groups from diazoalkanes and organic azides.5
Many MTO-catalyzed oxidations of hydrogen peroxide
have been reported, including the oxidations of alk-
enes,1,6,7 cobalt thiolates,8 organic sulfides,9 anilines,10
alkynes,11 and phosphines.12 We note this series of
results, not because hydrogen peroxide is in any way
involved with the transformations of alcohols described
in this paper but because the precursors to the rhenium
peroxide intermediates from MTO and H2O2 can reason-
ably be used as models for them.
The dehydration of alcohols provides an important
means of preparing ethers. The Williamson ether syn-
thesis,13 one of the most widely used procedures, calls
for the initial conversion of alcohols to halides or tosy-
lates. Other synthetic methods have been reported, but
they are not without limitations.14-18 The method de-
veloped in this work is a direct one.
Amines are of considerable practical importance, find-
ing use as antioxidants in fuel oils, rubber stabilizers,
medicinal drugs, detergents, and herbicides.27 Generally,
the alcohol used to form an amine must first be converted
to a halide. The direct methods so far reported for the
catalytic amination of alcohols require co-catalysts, such
as these: CuO/γ-Al2O3,28 Al(OBut)3/Raney Ni,29 CuO/
Cr2O3/Na2O/SiO2/H2O,27 RuCl2(PPh3)2/Ph3P,30 and Ph3P+-
NMeC6H4 I-/BunNHMe/DMF.31 We have developed a
simpler procedure in which MTO is the sole catalyst.
Disproportionation of alcohols requires hydride trans-
fer and is usually done with Al2O3 at >300 °C32-34 or over
Another important transformation of alcohols is an
elimination reaction to yield olefins. Known methods
(17) Noda, I.; Horita, K.; Oikawa, Y.; Yonemitsu, O. Tetrahedron
Lett. 1986, 27, 1917.
(18) Kim, S.; Chung, K. N.; Yang, S. J . Org. Chem. 1987, 52, 3917.
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707.
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(22) Gaston, J . L.; Grundon, M. F.; J ames, K. J . J . Chem. Soc.,
Perkin Trans. 1 1980, 1136.
(23) Piers, E.; Karunaratne, V. Can. J . Chem. 1989, 160.
(24) Posner, G. H.; Roskes, E. M. S.; Chang, H. O.; Carry, J . C.;
Green, J . V.; Clark, A. B.; Dai, H.; Anjeh, T. E. N. Tetrahedron Lett.
1991, 6489.
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Synthesis 1994, 685.
(26) Dorta, R. L.; Suarez, E.; Betancor, C. Tetrahedron Lett. 1994,
35, 5035.
(27) Baiker, A.; Richarz, W. Tetrahedron Lett. 1977, 18, 1937.
(28) Baiker, A.; Richarz, W. Synth. Commun. 1978, 8, 27.
(29) Botta, M.; Angelis, F. D.; Nicoletti, R. Synthesis 1977, 722.
(30) Watanabe, Y.; Tsuji, Y.; Ohsugi, Y. Tetrahedron Lett. 1981, 22,
2667.
X Abstract published in Advance ACS Abstracts, December 15, 1995.
(1) Herrmann, W. A.; Fischer, R. W.; Rauch, M. U.; Scherer, W. J .
Mol. Catal. 1994, 86, 243.
(2) Herrmann, W. A.; Wagner, W.; Flessner, U. N.; Volkhardt, U.;
Komber, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1636.
(3) Herrmann, W. A.; Roesky, P. W.; Wang, M.; Scherer, W.
Organometallics 1994, 13, 4531.
(4) Zhu, Z.; Espenson, J . H. J . Mol. Catal. 1995, in press.
(5) Zhu, Z.; Espenson, J . H. Submitted for publication.
(6) Herrmann, W. A.; Fischer, R. W.; Scherer, W.; Rauch, M. H.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1157.
(7) Al-Ajlouni, A.; Espenson, J . H. J . Am. Chem. Soc. In press.
(8) Huston, P.; Espenson, J . H.; Bakac, A. Inorg. Chem. 1993, 32,
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(10) Zhu, Z.; Espenson, J . H. J . Org. Chem. 1995, 60, 1326.
(11) Zhu, Z.; Espenson, J . H. J . Org. Chem. 1955, 60, 7090.
(12) Abu-Omar, M. M.; Espenson, J . H. J . Am. Chem. Soc. 1995,
117, 272.
(13) Barton, D.; Ollis, W. D. Comprehensive Organic Chemistry;
Pergamon: Elmsford, 1979; Vol. 1, p 819.
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(15) Traynellis, V. J .; Hergenrother, W. L.; Hanson, H. T.; Valicenti,
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(16) Emert, J .; Goldenberg, M.; Chiu, G. L.; Valeri, A. J . Org. Chem.
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0022-3263/96/1961-0324$12.00/0 © 1996 American Chemical Society

Organic Reactions Catalyzed by Methylrhenium Trioxide
J . Org. Chem., Vol. 61, No. 1, 1996 325
Ta ble 1. Yield sa of Sym m etr ic Eth er s a n d of Oth er P r od u cts F or m ed by Alcoh ol Deh yd r a tion Ca ta lyzed by
Meth ylr h en iu m Tr ioxid e
R1R2CHOH
R1
)
R2
)
convn %
yield of ether, %
other products
Ph
Ph
Ph
Ph
H
Me
Et
Ph
Ph
36
86
89
100
100
30
80a
79b
100
90
∼3% PhCHO, ∼3% PhMe
∼2% PhC(O)Me, ∼2% PhEt
∼3%PhC(O)Et, ∼3% PhCH2Et
4-MeC6H4
∼5% 4-MeC6H4C(O)Ph
∼5% 4-MeC6H4CH2Ph
4-ClC6H4
4-MeC6H4
4-ClC6H4
H
10
42
10
34
∼4% 4-MeC6H4CHO
∼4% 4-MeC6H4CH3
4-MeOC6H4
H
48
36
∼6% 4-MeOC6H4CHO
∼6% 4-MeOC6H4CH3
∼2% disproportionation
∼2% disproportionation
1-naphthyl
2-naphthyl
n-C4H9
n-C5H11
n-C6H13
H
H
H
H
H
∼4
∼4
∼7
∼8
∼8
a
b
Yields of the ethers were based on the alcohols for reactions carried out in benzene. Two isomers, in a 1:1 ratio.
HY-zeolite in refluxing carbon tetrachloride.35 We have
been able to cause disproportionation to occur at room
temperature in benzene with a catalytic amount of MTO.
The shortcoming of the MTO procedures, however, is
that the various transformations of alcohols take place
concurrently and competitively, controlled largely by the
structure of the alcohol. This aspect of the chemistry will
be evident from the results obtained.
Ta ble 2. F or m a tion of Un sym m etr ic Eth er s by Cou p lin g
Ar om a tic a n d Alip h a tic Alcoh ols, Ca ta lzyed by MTO
yield,
a
aromatic alcohol aliphatic alcohol
%
other product
Ph2CHOH
Ph2CHOH
Ph2CHOH
Ph2CHOH
Ph2CHOH
EtOH
89
91
92
95
10
69
85
(Ph2CH)2O, 11%
(Ph2CH)2O, 9%
(Ph2CH)2O, 8%
(Ph2CH)2O, 5%
(Ph2CH)2O, 90%
(PhMeCH)2O, 31%
(PhMeCH)2O, 15%
n-C3H7OH
n-C4H9OH
n-C5H11OH
tert-Me3COH
PhCH(OH)CH3 EtOH
PhCH(OH)CH3 CH2dCHCH2OH
Resu lts
a
Yields are based on the limiting amount of the aromatic
alcohol, 10 mmol, dissolved in 15 mL of the dry aliphatic alcohol
to which 0.2 mmol of MTO was added.
F or m a tion of Eth er s. Primary aliphatic alcohols in
benzene gave low yields after 2 days: n-C4H9OH, ∼7%;
n-C5H11OH, ∼8%; n-C6H13OH, ∼8%. Further time did
not increase the yield, as the process is limited by the
buildup of water in the system, evidently implying a
competition between the two hydroxylic reagents. The
conversion of alcohols can be represented by this equa-
tion:
amounts. Increasing the size of the aryl group decreased
the conversion; with 1-naphthyl and 2-naphthyl groups,
only 6% conversion was found. Table 1 summarizes these
data.
Inhibition by electron-withdrawing substituents re-
flects a powerful and remarkable electronic effect. It is
not entirely eliminated with the use of electron-donating
substitutents, as one might suppose, very likely because
of the buildup of water, as noted previously.
When low molecular weight primary alcohols were
used as the solvent, the aromatic alcohols reacted with
them in the presence of MTO to form unsymmetric ethers
in high yields, 69-95%. With a tertiary alcohol, however,
such as tert-butyl alcohol, only 10% of the aromatic
alcohol was converted to an unsymmetric ether under
these conditions, the balance being the ether formed by
the self-coupling of the aromatic ether. The results are
given in Table 2.
The successful strategy for unsymmetric aromatic
ethers is based on using the more reactive aromatic
alcohol at a much lower concentration than the aliphatic
alcohol. Even then, some of the symmetric ether from
the aromatic alcohol was formed. If the two alcohols
used had similar reactivities, such as PhCH(OH)Me
and PhCH(OH)Et, then three ethers were formed,
(PhCHMe)2O, (PhCHEt)2O, and the unsymmetric ether
PhCHMeOCH(Ph)Me. As it happened, the three were
nearly equal in yield. In all the other cases, however,
the unsymmetric ether was the major product. The
findings are summarized in Table 3.
2 RCH2OH MTO8 RCH2OCH2R + H2O
(1)
The situation might be improved by employing a dehy-
drating agent, as it was in olefin-forming reaction
referred to in the next section, but that prospect was not
explored here.
Aromatic alcohols, especially secondary ones, gave
higher conversions and greater yields of ethers. The
addition of MTO to solutions of such alcohols in benzene
gave rise to a yellow color; the nature of this intermediate
will be considered subsequently. For alcohols of the
general formula PhCH(OH)R, the conversion increased
with the size of the group R; thus R ) H, 36%; Me, 86%;
Et, 89%; Ph, 100%. If the aryl group has an electron-
withdrawing group attached, such as 4-NO2, 4-Br, or 4-Cl,
then no ether was formed, and the starting material
remained unchanged. Although (4-ClC6H4)2CHOH has
two electron-withdrawing groups, it was converted to the
ether in 10% yield with MTO in benzene. On the other
hand, electron-donating groups improved the reaction
only to a mild extent: for 4-XC6H4CH2OH, the yields are
R ) H, 36%; Me,42%; R ) MeO, 48%. In these cases
meso and racemic ethers were produced in nearly equal
(34) J ayamani, M.; Murugasen, N.; Pillai, C. N. J . Catal. 1984, 85,
527.
(35) Climent, M. J .; Corma, A.; Garcia, H.; Iborra, S.; Primo, J . J .
Recl. Trav. Chim. Pays-Bas 1991, 110, 275.
Olefin s fr om Alcoh ol Deh yd r a tion . The process in
eq 2 was observed for the aliphatic alcohols, but the yields
were quite low. With aromatic alcohols, however, the

326 J . Org. Chem., Vol. 61, No. 1, 1996
Zhu and Espenson
Ta ble 3. Yield s of Un sym m etr ic Eth er s fr om P a ir s of
Ar om a tic Alcoh ols, Ca ta lyzed by MTO
Ta ble 4. MTO-Ca ta lyzed Deh yd r a tion of Alcoh ols To
F or m Olefin s
a
Relative to the alcohol taken in limiting amount. 10 mmol of
the limiting alcohol was used, with 0.2 mmol of MTO in 100 mL
of dry benzene.
olefin yields were satisfactory, although accompanying
amounts of ether and disproportionation products were
formed. The data are given in Table 4. For the aliphatic
alcohols, the solvent as well as the reagent, the data are
given in terms of catalytic turnovers, rather than yields.
As the yield of olefins formed is low for some of these
alcohols, the term “turnover” was used to describe
reaction. For example, dehydration of 15 mL (124 mmol)
of PhCH(OH)Me by 0.2 mmol of MTO in 3 days yields
20 mmol of styrene; no solvent was used.
a
MTO (0.2 mmol) was dissolved in 15 mL of alcohol and allowed
b
to stand for 3 days. Turnovers ) mol of product/mol of catalyst,
after 3 days. c The alcohol (20 mmol) and MTO (0.2 mmol) were
dissolved in 100 mL of dry benzene and allowed to stand for 3
d
days. Isolated yield.
Am in ation of Ar om atic Alcoh ols. Secondary amines
were obtained from primary aromatic alcohols and
aliphatic or aromatic amines as in eq 5. These prepara-
tions were carried out by using the alcohol in excess in
dry benzene. The yields (based on the limiting amines)
ArCH(OH)R + R′NH2 cat. MTO8 ArCHRNHR′+ H2O
Some polymer formation, also MTO-catalyzed, ac-
companied these reactions. A solution of 0.2 mmol of
MTO in 25 mL of the alcohol was sealed in a 30-mL glass
vial for 1 month, at which time 11% of the alcohol had
been converted to the polystyrene. Although attempts
to synthesize the ether from 2,3-dimethyl-1-phenyl-1-
propanol failed, an olefin was obtained after dehydration
and rearrangement:
(5)
were satisfactory, although considerable quantities of the
ethers were also obtained. When the alcohol and amine
were taken in equal quantity, however, the yields of
secondary amine were considerably reduced. The data
are summarized in Table 5.
Disp r op or tion a tion of a r om a tic a lcoh ols, cata-
lyzed by MTO, was observed for all the primary and
secondary alcohols, except benzhydrol and those alcohols
with an electron-withdrawing group. These studies
were carried out with 10 mmol of alcohol and 0.2 mmol
of MTO in 100 mL of dry benzene. 4,4′-Dimethoxyben-
zhydrol and 9-hydroxyxanthene undergo disproportion-
ation only, eq 6; no ether was formed.
The tertiary aromatic alcohol, 1,2-diphenyl-2-propanol,
yielded two olefins, (Z)- and (E)-methylstilbenes, in a 1:5
ratio:
AR1AR2CHOH cat. MTO8 Ar1Ar2CdO + Ar1Ar2CH2
(6)
By way of comparison, dehydration with sulfuric acid,
which occurs through a pure E1 mechanism, gave a Z:E
ratio of 1:18.36
(36) Crummit, O.; Becker, E. I. Organic Syntheses; Wiley: New York,
1963; Collect. Vol. 1, p 771.

Organic Reactions Catalyzed by Methylrhenium Trioxide
J . Org. Chem., Vol. 61, No. 1, 1996 327
Ta ble 5. Yield s of Secon d a r y Am in es fr om
MTO-Ca ta lyzed Am in a tion s of Alcoh ols, eq 5
aromatic alcohols proceeds through similar intermedi-
ates, with a single alcohol, 5, and possibly with two, 6.
The second of these is less certain, since ethers could
result instead from the attack of a second alcohol on 5.
alcohol
Ph2CHOH
amine
PhNH2
yield, %a
91 (26)b (Ph2CH)2O
other products
9-xanthenol
(MeOC6H4)CHOH
(MeC6H4)PhCHOH PhNH2
(MeOC6H4)CHOH
Ph2CHOH
PhNH2
PhNH2
96
>95
>95
disproportionation
disproportionation
MeC6H4CHOPh
disproportionation
(Ph2CH)2O
n-C6H13NH2 94
n-C6H13NH2 92
a
Based on the amine in a reaction of 1 mmol of amine and 3
mmol of alcohol in 20 mL of dry benzene to which 0.2 mmol of
b
MTO was added. When
benzhydrol were used.
1 mmol of amine and 1 mmol of
4-Methoxybenzhydrol gave both disproportionation
(60% of the starting alcohol) and ether formation (40%).
Monoaryl alcohols, ArCH2OH, underwent disproportion-
ation to a much smaller extent: Ar ) Ph, 6%; 4-MeC6H4,
8%; 4-MeOC6H4, 12%; 1-naphthyl, 2%; 2-naphthyl, 2%.
Aryl alkyl alcohols gave these percentages of dispropor-
tionation: PhCH(OH)Me, 4%; PhCH(OH)Et, 6%; 1,2,3,4-
tetramethyl-1-naphthol, 11%.
Attempts to isolate 5 and 6 were not successful; this
is not unduly discouraging, however, since the OH
analogues have not been directly seen. As an indepen-
dent precedent, we note the condensation reaction be-
tween MTO and 1,2-dihydroxybenzene.38
Disproportionation was also observed between two
different alcohols, 9-hydroxyxanthene and excess PhCH-
(OH)Et, which yielded only one set of products when the
reaction was carried out in dry benzene, eq 7. No mixed
ether was formed.
Certain Lewis acids, such as zinc chloride,18 catalyze
the formation of ethers from alcohols, but these are really
stoichiometric reactions that are critically dependent on
the solvent; the zinc chloride reaction, for example, is
successful only in dichloroethane. MTO appears to be the
first transition metal complex that leads to the direct
formation of ethers from alcohols when used in catalytic
amounts; this reaction can be carried out in benzene,
toluene, dichloromethane, chloroform, acetone, and in the
alcohols themselves.
It should also be noted that no EPR signal was
observed during the course of the disproportionation of
4,4′-dimethoxybenzhydrol, even when the spectrum was
recorded at 110 K in frozen C6H6.
Our findings that (a) no reaction occurred with alcohols
containing an electron-withdrawing group at the para
position such as NO2, Br, and Cl, (b) an olefin was formed
from 2,2-dimethyl-1-phenyl-1-propanol, and (c) alcohols
underwent disproportionation catalyzed by MTO lead us
to suggest that these reaction occur through a carbocation
intermediate (7).
Discu ssion
The intermediates characterized and inferred during
(a) the exchange of oxygen atoms between water and
MTO and (b) the formation of the rhenium peroxides CH3-
Re(O)2(η2-O2), A, and CH3Re(O)(η2-O2)2(H2O), B, may be
pertinent to the present work. Intermediate 1 has been
characterized as a precursor to A,37 and it thus seems
highly likely that intermediate 2 serves as the vehicle
for oxygen exchange between MTO and water. In both
cases, nucleophilic attack by H2O2 or H2O on the highly
electropositive Re(VII) center, as shown in 3, will gener-
ate the intermediates 1 and 2.
Two possible mechanisms can be suggested on the
basis of the results obtained. One of these is a concerted
process. In it, the formation of the dialkoxide 6 is the
first step, and this yields the ether, as in eq 10:
6 f MTO + R-O-R
(10)
This mechanism also explains why the smaller primary
alcohols can form ethers, whereas the larger ones can-
not: 1-dodecanol and 1-undecanol yield only the alkene
by dehydration. This is due to the limited space in the
coordination sphere of MTO. The dehydration reaction
of 1,2-diphenyl-2-propanol should give a Z:E ratio of
about 1:1 if the reaction is fully concerted. The observed
ratio of Z:E ) 1:5 suggests that both concerted and
carbonium ion pathways may contribute.
As far as the carbonium ion mechanism is concerned,
formation of 6 is not necessary. After the first alcohol
has been added, giving 5, a carbocation intermediate (7)
can be formed. Indeed, the ethers and the amines are
formed by the nucleophilic addition of a second alcohol
or of the amine to 5.
From these comparisons, and since the rhenium diper-
oxide B is yellow, like the peroxorhenium complex, it is
logical to infer that the reaction between MTO and
(37) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 1638.
(38) Takacs, J .; Kiprof, P.; Riede, J .; Herrmann, W. A. Organome-
tallics 1990, 9, 782-787.

328 J . Org. Chem., Vol. 61, No. 1, 1996
Zhu and Espenson
1:1) as the eluant. Alternatively, toluene, dichloromethane,
chloroform, acetone, and the alcohols themselves can be used
as solvent.
Un sym m etr ic Eth er s. The alcohol (10 mmol) and MTO
(0.2 mmol) were dissolved in 15 mL of the dry aliphatic alcohol
and allowed to stand at room temperature for 2 days. The
crude products were obtained by removing the excess aliphatic
alcohol and the symmetric ether using rotary evaporation. The
pure ether was isolated by column chromatography using
hexane/ethyl acetate (10:1 to 1:1) as the eluant. When a pair
of unsymmetric aromatic alcohols was used, the procedure was
the same except that the two alcohols were used in 20 mL of
dry benzene, the more reactive alcohol being taken in limiting
quantity, 10 mmol, along with 0.2 mmol of MTO.
Am in a tion Rea cton s. The alcohol (3 mmol), amine (1
mmol), and MTO (0.2 mmol) were dissolved in 20 mL of dry
benzene and allowed to stand for 2 days. The balance of the
procedure was the same.
Olefin F or m a tion . The catalyst (0.2 mmol) was dissolved
in 15 mL of the dry alcohol and allowed to stand for 3 days.
The olefins were isolated by distillation. For some of the
aromatic alcohols, dry benzene with 20 mmol of alcohol and
0.2 mmol of MTO was used.
Id en tifica tion s. The products were identified by their
NMR spectra, Tables S-1 and S-2, in comparison with litera-
ture data.39,40
Those alcohols that can generate a stable carbocation
allow a second alcohol to react with it; this yields a ketone
and and an alkane through disproportionation. Alter-
natively, the cation can be intercepted by the alcohol or
by the amine. The results we have obtained suggest that
the concerted process of eq 11 is the dominant one for
the primary aliphatic alcohols, whereas a carbocation
intermediate provides the major pathway for the aro-
matic alcohols. Because an aromatic alcohol can form a
more stable carbocation intermediate than an aliphatic
alcohol, higher conversions were observed for the aro-
matic alcohols.
Exp er im en ta l Section
Ack n ow led gm en t. This research was supported by
the U.S. Department of Energy, Office of Basic Energy
Sciences, Division of Chemical Sciences under contract
W-7405-Eng-82.
Ma ter ia ls. The chemicals were purchased commercially,
and their purity was verified by GC-MS. The samples of
methylrhenium trioxide used in this research were also
purchased (Aldrich). The solvents were purified by standard
procedures.39
Sym m etr ic Eth er s. Gen er a l P r oced u r e. The alcohol
(40 mmol) and MTO (0.2 mmol) were dissolved in 100 mL of
dry benzene, and the solution was allowed to stand at room
temperature for 2 days. Much of the solvent was removed by
rotary evaporation. Separation and purification of the ether
was realized by vacuum distillation, recrystallization, or
column chromatography using hexane/ethyl acetate (10:1 to
Su p p or tin g In for m a tion Ava ila ble: Tables of NMR data
for all of the reaction products (6 pages). This material is
contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J O951613A
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